Answers

Hi FHtodd and welcome to Goy. Well in checking that I was about to give you the correct info I've discovered that 'mares tail' is the one that grows in water yours is 'horse tail'. Obviously it would be a lot easier if you could dig as much as possible up. As you can't next spring once it is part way grown trample on the stems and apply roundup or similar. Keep repeating and eventually you will get rid of most of it.

For a laugh (huh) one day, I decided to follow the threadlike black rhizome as far down as I could into our clay. I got to 2ft, straight down, before I found a thicker piece of stem, and a junction, and other stems growing up. They say it goes down 4ft or more. In the right soil, I can believe it. It fixes silica in the stems, and used to be used for scouring pots and pans. It does a good job on flower pots, but I refuse to promote its growth for that! Phil

At nearly £30 a bottle I will stick to just pulling it up Anchorman. It thrives in poor soil and I have spent the last 12 years improving my soil and pulling it up when it appears, this year I have had hardly any.

It is expensive but it dilutes doen to 20 litres so works out cheaper than the litre stuff you buy in the shops for 2.99+ a bottle. I've split the cost with several friends so it hasn't cost me more than about £6 and £2's worth has completely eradicated marestail from my third of an acre garden

I fully respect those who are prepared to weed and weed and weed for in your case 12 years. Most people just lose the will to live! :)

My elderly mum and 4 elderly customers have spent hours and hours a year pulling the stuff out and it is back again in weeks.

One spray with this stuff and it is gone. The relief particularly on my mum's face was a joy to see.

I Cannot believe any gardener does not weed Anchorman be it Ground elder, nettles or Mares tail plus what the birds, animals or wind plant its all part of maintaing a garden and I love a good weeding session. I have yet to be as easily convinced as you are that its gone after one application! Sorry but this sounds more like the hyped claims of the manufacturer which I believe they have yet to make/prove? If it does not come back next spring I will be very surprised as I understand it lurks down as low as 4-5 feet under the soil. It has a habit of coming up in the centre of a plant especially in rockeries.
Finally Mares tail is actually an aquatic weed (Hippuris vulgaris) which is commonly found in ponds or slow flowing streams So perhaps improving the drainage on your land as I have may help?